Serpins (serine proteinase inhibitors) comprise a superfamily of proteinase inhibitors with a unique mechanism of irreversible inhibition, together with non-inhibitory homologs. Previous studies identified seven serpins in thiol extracts of bread wheat flour visualized via native PAGE. Building from these identified and partially characterized serpins, we combined a large number of expressed sequenced tags with reads from the cultivar Chinese Spring 454 NGS sequencing project to build serpin contigs. Using data from the recently completed wheat genome project (IWGSC RefSeq v1.0, cv Chinese Spring) and barley genome project (IBSC 2017, cvs Morex, Barke and Bowman), we propose a systematic classification of the complement of serpins in wheat and barley. In wheat, a total of 52 loci, consisting of 19 subfamilies, were classified into four distinct groups based on protein phylogeny and a high degree of reactive center conservation. Additional serpin sequences and pseudogenes were retrieved from the four genomes (wheat A, B, D and barley H) and expressed serpin sequences (ESTs) to support our model of serpin evolution and the proposed nomenclature. One of the subfamilies, SZx, is suggested to represent an ancestral serpin with conserved inhibitory specificity in plants. Expression of the serpin genes varies spatially and temporally, with some genes expressed specifically in the grain at very high levels, and others expressed ubiquitously at low levels. Evolution of wheat has resulted in some genes for which all three homoeologs are active and others for which one, two or all homoeologs are inactive. The SZ1 to SZ5 serpins, which are highly expressed in grain endosperm, are specific to Pooideae cereals including wheat, barley, oat, and rye. Our classification system can be extended to other grass serpins with few, if any, adjustments required. This knowledge will help to elucidate the physiological roles of wheat serpins and their contributions to agronomic and grain quality traits of this globally important crop.